This invention relates to thermal stencil master sheets for stencil printing and methods for their production. These thermal stencil master sheets comprise a thermoplastic resin film adhered to a porous substrate with an adhesive.
Thermal stencil master sheets provide stencils when placed in contact with a thermal print head of a thermal transfer printer or the equivalent thereof. The print head is activated as the thermal stencil master sheet is scrolled by, creating perforations therein. These perforations permit the flow of ink from a mesh printing drum to a receiving substrate, creating a printed image that conforms to the stencil pattern.
Representative disclosures in the art of thermal stencil master sheets are as follows:
U.S. Pat. No. 4,628,813, issued to Hasegawa et al., describes a stencil duplicator which prepares a stencil master and prints with the stencil master. The means for making the stencil master includes apparatus for selective illumination with light.
U.S. Pat. No. 4,961,377, issued to Bando et al., describes a thermal stencil master sheet which employs a urethane adhesive.
U.S. Pat. No. 5,160,564, issued to Hasegawa et al., describes a process for producing a thermal stencil master sheet which protects the film from breakage by laminating thermoplastic films onto the surfaces of a base film, bonding porous substrates to the thermoplastic films and subsequently stripping the thermoplastic films with the porous substrates from the base film. The adhesives disclosed are vinyl acetate adhesives, acrylic adhesives, polyester adhesives, urethane adhesives, epoxy adhesives and EVA adhesives.
U.S. Pat. No. 5,243,906, issued to Okusawa, describes a thermal stencil master plate of a porous support and thermoplastic resin film.
U.S. Pat. No. 5,245,932, issued to Ujiie, describes a high resolution thermal stencil master sheet which uses different zones (contacting zones and clearance zones) on the thermoplastic resin. An "ultraviolet curing type" adhesive is described at column 4, lines 45-46.
U.S. Pat. No. 5,373,785, issued to Yamamoto et al., describes a mimeographic transfer printing machine, wherein ink is forced through the stencil supported on a stencil supporting drum.
U.S. Pat. No. 5,415,090, issued to Natori et al., describes a method for manufacturing a print master by forming perforations in a thermosensitive stencil paper.
U.S. Pat. No. 5,438,347, issued to Shishido et al., describes a device for making a thermal stencil master sheet within a stencil printer.
U.S. Pat. No. 5,450,789, issued to Hasegawa, describes a stencil printing method which uses a plurality of stencil master plates which can be superimposed during printing to achieve color printing.
U.S. Pat. No. 5,513,565, issued to Hasegawa, describes a stencil printing device with a plurality of printing drums for printing full color images.
U.S. Pat. No. 5,517,913, issued to Oshio et al., describes a stencil printing device which uses a sensor for detecting an ink type to vary the pressing force of a press roller.
U.S. Pat. No. 5,522,313 issued to Okusawa describes thermal stencil master plates of a porous support and thermoplastic resin film, wherein an unprocessed portion includes swelled and solidified lumps of thermoplastic resin film that avoids the expansion of the perforations.
U.S. Pat. No. 5,526,032 issued to Nakamura describes a method for processing a stencil master plate using a thermal head, wherein the perforations are controlled.
The thermal stencil master sheets are typically obtained by laminating a thermoplastic resin film onto a porous substrate such as a porous thin paper with or without an adhesive. A release layer is typically provided on the surface of the thermoplastic resin film to prevent adhesion of the film to the print head when perforated and the adhesion of the film to the receiving substrate during use. The thermoplastic resin films used for the thermal stencil master sheets have varied to a limited extent. Polyester films, films of propylene copolymers and vinylidene chloride-vinyl chloride copolymer films have been found to be suitable. The composition of the porous substrate can vary widely and is said to include thin papers of both natural and synthetic fibers. The adhesive has many requirements which include the following:
1. the adhesive must permit the coating process to be carried out rapidly; PA1 2. the adhesive must melt with the thermoplastic resin film when forming perforations; and PA1 3. the adhesive must be resistant to solvents within the printing ink. PA1 a) forming a laminate of a porous substrate, a layer of a curable liquid epoxy/coreactant formulation having a thickness ranging from 0.3-2.5 gms/m.sup.2, and a thermoplastic resin film, PA1 b) applying heat and pressure to the laminate to cure the curable epoxy/coreactant formulation therein to obtain a solid adhesive layer, said curable liquid epoxy/coreactant formulation comprising:
The adhesives presently being used primarily include vinyl acetates, acrylics and rubbers, although Hasegawa et al. (U.S. Pat. No. 5,160,564) additionally mention polyesters, urethane, epoxy and EVA adhesives are suitable. Various deficiencies of these particular adhesives include the need for solvents and dispersing agents. These components reduce productivity by lowering line speeds due to drying and solvent elimination.
The use of organic solvents complicates compliance with environmental regulations and restrictions and also adds to the cost in that the solvent removed must be captured and/or incinerated. Suitable water-based adhesives have not been achieved and the use of hot melt adhesives is expected to cause problems with the thin thermoplastic resin film.
Another deficiency is the poor adhesive strength exhibited by some adhesives. Vinyl acetate adhesives have been widely used due to their ease of handling. However, higher adhesive strength is desired. The thermal stencil master sheets with this adhesive suffer damage when in contact with the printing ink over an extended period, resulting in unclear printed images.
Alternatives to the vinyl acetates have been proposed, such as the urethane adhesives containing a urethane prepolymer as disclosed in U.S. Pat. No. 4,961,377. These adhesives are said to take from 24-48 hours to cure at room temperature following the addition of moisture (see column 4, lines 23-26). In addition, solvents are said to be employed when necessary to obtain a uniform coating. While these urethane adhesives accelerate the laminating process, complex procedures are required to prepare and use the urethane prepolymer. It is desirable to provide thermal stencil master sheets having an adhesive which is simple to prepare and use and provides stencils with high endurance.
Epoxy adhesives are known for their strong adhesion to a variety of materials such as glass, metal and fibers. The use of epoxy adhesives to form stencil master sheets has been mentioned by Hasegawa et al. However, solid thermoplastic epoxies still require the use of solvent. Curable liquid epoxy formulations provide advantages over solvent or aqueous based adhesive coatings in that there is no need to dry a curable liquid coating or capture/incinerate any organic solvents therefrom. Typically, all of the curable liquid epoxy coating formulation forms the final coating and there are no losses from the evaporation of solvent. This simplifies compliance with environmental regulations. However, curable liquid epoxies typically cure to solid thermosets by crosslinking through the oxirane ring groups and/or hydroxyl groups. These crosslinks and thermosetting properties can interfere with the flow properties of the adhesive such that it will not provide accurate perforations when exposed to a thermal print head. Such an adhesive will interfere with the flow of ink to a receiving substrate.
It is desirable to develop a thermal stencil master sheet and method for producing the same, wherein a curable liquid adhesive provides high adhesion between the porous substrate and thermoplastic film without interfering with the formation of perforations within the stencil or the flow of ink from a mesh printing drum to a receiving substrate.